The present invention relates to biological wastewater treatment processes including secondary clarification. More particularly, but without limitation thereto, the present invention relates to activated sludge wastewater treatment processes, and especially to those activated sludge wastewater treatment processes employing oxygen-enriched air or substantially pure oxygen for aeration of the material in the reactor of the process.
Activated sludge wastewater treatment processes in general terms employ a reactor in which a suspension of various microorganisms aerobically biodegrade dissolved and colloidal organics in the wastewater flowing into the process, with oxygen being continuously supplied to the suspension in the form of air, oxygen-enriched air, or pure oxygen. A downstream settling tank or secondary clarifier gravitationally separates the flocculated microorganisms from the treated wastewater. A portion of the suspension recovered from the secondary clarification step of the process is recycled to the reactor, and the remainder is passed through a sludge treatment process to remove any remaining water from the sludge and to render the sludge suitable for disposal by landfilling, for example, or for further use, e.g., as a fertilizer.
A species of an activated sludge process which has been briefly mentioned above aerates the reactor with pure or substantially pure oxygen. This sort of process, sometimes referred to in industry as Union Carbide's "Unox" process, is very often employed where space for the activated sludge reactor or reactors (typically there are several reactors in series) is limited or where there is a desire to reduce capital costs and a supply of oxygen is on hand, since the pure oxygen process supplies the same amount of oxygen as a conventional process using ordinary air, in roughly one-fifth the volume required in a conventional process. The reactors in a pure oxygen activated sludge process are thus typically small compared to the reactors in other activated sludge processes, and have a much shorter residence time for materials passing through the reactors.
Because of the much shorter residence times and higher throughputs associated with the pure oxygen activated sludge treatment processes, those changes in the character of the influent wastewater which could adversely affect the system's microorganisms or the secondary clarification are not as easily counteracted and can be more critical in their effects. Relatively minor changes in the character of the incoming wastewater which would have little adverse effect in other processes because of a dilution factor in a large reactor, or which could be corrected over time with little damage to the overall process or to the waters in which the treated effluent is to be discharged, become much more troublesome in a pure oxygen activated sludge process.
For this reason, a thorough understanding and appreciation of the effect on the overall process of changes of various magnitudes in the character and composition of the incoming wastewater stream becomes critical, if damage either to the process or to the environment is to be avoided.